1. Field of the Invention
This invention relates to apparatus and a method for generating a test signal or use in determining the performance of a receiver.
2. Description of the Prior Art
An important measure of a receiver's performance is its sensitivity and one means for measuring this sensitivity is to compare the measured bit error rate (BER) of a received signal with the signal to noise ratio.
The main factors that effect the measured BER for a given receiver are channel noise (e.g. thermal noise, noise of other transmitted signals and interference from other communication channels) and multipath propagation. Of these two effects multipath propagation can, if not compensated for, have the most significant effect on BER.
Multipath propagation occurs where two or more different propagation paths exist between a transmitter and a receiver. For example, a second path may come into existence if a reflective surface (e.g. from a nearby building) provides an alternative path between the transmitter and receiver. The combination of the two paths at the receiver causes frequency-dependent attenuation, frequency-dependent phase nonlinearity and delayed response effects.
The channel effects resulting from multipath propagation vary depending on whether the channel is narrow or wide band.
Multipath propagation in a narrowband channel can result in fast fading characteristics when the transmitter/receiver or medium is in motion. The rate of fading is proportional to the velocity between the transmitter and receiver.
Multipath propagation in a wideband channel results in frequency selective fading (i.e. the signal fading at different frequencies within the channel vary independently).
The fading (i.e. a varying increase/decrease in signal strength) occurs due to constructive and destructive interference between the multipaths.
Ideally the BER measurement for a receiver would be conducted under realistic real life conditions. However, to do so would require a fully designed receiver operating under a variety of different conditions (e.g. in different types of area, with the receiver moving at different velocities).
However, to simplify the BER measurement process and to allow the receiver performance to be tested at an early stage of the design, various models have been developed for modeling the channel propagation characteristics between a transmitter and a receiver. The use of these models enables a receiver's performance to be determined for different channel propagation conditions in a laboratory at an early stage of the receiver's design and for field testing a receiver (e.g. type approval test of products).
Additive White Gaussian Noise (AWGN) is used to model the channel noise. AWGN has a constant energy density independent of frequency and closely matches the measured noise on a channel.
Typically a receiver's performance will decrease as the channel noise increases (i.e. the measured BER of a receiver will increase as the channel noise increases).
The fading effects resulting from multipath propagation are modeled using either a Ricean or Rayleigh probability density function.
A Ricean fading model is typically used for modeling a channel where the line of sight path is dominant. A Rayleigh fading model, a special case of the more general Ricean model, is typically used for modeling a channel that does not have a dominant propagation path.
Using the channel fading models wireless receiver designers are able to model channel characteristics for a variety of different conditions (e.g. different speeds at which a mobile receiver may be moving, urban or city environments) in a laboratory. Accordingly, these channel models can be used to determine the performance of a receiver over a variety of different conditions at an early stage of the receiver's design and for field testing a receiver (e.g. type approval test of products).
Some channel models will cause bit errors to occur in a receiver when modeling long lasting fades. However, for a receiver having a power control RF link the receiver's power control mechanism will in operation work to reduce these fades. Therefore these channel models will give pessimistic performance estimates compared to actual measured results.
It would be desirable to improve this situation.